A water gas shift reaction is an important reaction step in many chemical processes. Particularly, when fossil fuels such as hydrocarbons or alcohols are reformed to produce hydrogen for use as fuel in fuel cells, which have recently been of great interest, carbon monoxide contained in reformate gas can be converted into carbon dioxide in the presence of water vapor, so that the amount of carbon monoxide acting to reduce the performance of an electrode catalyst in fuel cells (particularly, polymer electrolyte fuel cells), as shown in the following reaction equation, can be reduced and, at the same time, the production of hydrogen can be increased:CO2+H2OCO2+H2, ΔH=−41.1 kJ/mol
The water gas shift reaction is widely used in large-scale processes requiring a large amount of hydrogen, for example, the petrochemical industry, and catalysts thereof are well known. The reaction is a mild exothermic reaction and needs to be carried out at a low temperature in order to reduce the amount of carbon monoxide to a given concentration or less. Generally, two kinds of water gas shift reactors are used in commercialized processes.
The water gas shift reactions include a high-temperature water gas shift reaction, which is carried out at a high temperature of 350-450° C., and a low-temperature water gas shift reaction, which is carried out at a temperature of 190-260° C. [Catalyst Handbook 2nd Edition].
An iron-chromium oxide catalyst, which is used in the high-temperature water gas shift reaction, has an advantage in that it is easy to elevate the temperature of a catalyst bed during operation, but this catalyst shows low activity at 300° C. (generally, 350° C.) or below. A copper-zinc-alumina catalyst, which is used in the low-temperature water gas shift reaction, can treat carbon monoxide to a very low concentration at a temperature of 190-260° C. according to thermodynamic equilibrium, but is readily inactivated at a temperature higher than 250° C. and tends to be spontaneously oxidized upon exposure to oxygen, thus requiring special care in the replacement thereof. For this reason, the demand has arisen for a water gas shift reaction catalyst that can be used in a temperature ranging from 250° C. to 350° C., shows high activity, and does not show a severe decrease in the activity thereof, even upon exposure to oxygen.
In connection with this demand, Japanese Patent Laid-Open Publication No. 2004-000949, U.S. Pat. No. 6,846,475 and Japanese Patent Laid-Open Publication No. 2004-284912, for example, disclose platinum noble metal catalysts satisfying this demand. More recently, it has been reported that a catalyst, containing platinum as a main component and an active cocatalyst loaded on a support such as ceria, zirconia or titania, is particularly advantageous for the water gas shift reaction [Journal of catalysis 225 (2005) 327-336, Catalysis Today 99 (2005) 257]. Although it is well known that titania, ceria and zirconia are excellent as catalyst supports for platinum-based catalysts for the water gas shift reaction, these metal oxides are expensive, and it is very difficult to impart these metal oxides with high specific surface area, pore volume and mechanical strength, which are suitable for catalyst supports. Thus, in order to prepare a metal oxide support having such excellent properties, enormous expense is incurred.
Japanese Patent Laid-Open Publication No. 2000-178007, Korean Patent Laid-Open Publication No. 2004-0063130, and U.S. Pat. No. 6,723,298, for example, disclose methods of preparing catalysts in a more economic and commercially useful manner by adding cerium or zirconium to a metal oxide support such as alumina, which has large specific surface area, good pore development and high mechanical strength and, at the same time, is inexpensive.
However, these prior technologies either strictly limit a platinum precursor in order to increase the effect of adding cerium and the like [Korean Patent Laid-Open Publication No. 2004-0063130], or use a complex preparation process. Specifically, in order for platinum to be loaded on ceria more than on alumina, an excess amount of cerium is first loaded on a support such as alumina and then dried and calcined to form a layer of ceria (cerium oxide) on the surface of the alumina support. Thereafter, an aqueous platinum solution is additionally loaded on the ceria-containing alumina support and dried and calcined, thus preparing a catalyst [U.S. Pat. No. 6,723,298 and US Patent Publication No. 2002/0061277A1].
The reason why the limited precursor and the multi-step process are inevitably used in the prior art is that when a platinum precursor such as chloroplatinic acid (H2PtCl6.xH2O), which is strongly acidic and contains halogen, is used, the acidic precursor will be preferentially adsorbed on the base site of alumina and finally distributed mainly on the surface of alumina, so that the effect of addition of cerium on an activity promotion will be insignificant. In addition, it is known that halogen such as chlorine is attributable to a decrease in catalyst activity. To sufficiently obtain the effect of the addition of cerium for these two reasons, it is common to either excessively increase the content of platinum in order to increase the amount of platinum that is adjacent to cerium, or to use a platinum precursor not containing halogen such as chlorine [Korean Patent Laid-Open Publication No. 2004-0063130 and U.S. Pat. No. 6,846,475].
Meanwhile, oxygen storage materials, which have been used in the prior art, include ceria, zirconia, and ceria composites. A three-way catalyst for the purification of exhaust gas shows excellent conversion of carbon monoxide (CO), hydrocarbon, nitrogen oxide (NOx) and the like at a very narrow fuel-to-air ratio range of around about 14.6, but shows a significantly reduced conversion at an air-to-fuel ratio deviating from said range. Cerium has a very excellent ability of storing oxygen in the fuel lean zone and releasing oxygen in the fuel rich zone, because it is readily converted into Ce(III) and Ce(IV).
Thus, cerium has been adopted and used since the beginning of the 1990s, because it plays an important role in reducing the problem of a great decrease in conversion caused by a small change in the air-to-fuel ratio, when it is used together with the three-way catalyst. However, it is difficult to avoid exposing the three-way catalyst for the purification of exhaust gas to high temperatures, and in this case, ceria used in the three-way catalyst has a problem in that the fusion of micropores or the sintering of crystals occurs, resulting in a rapid decrease in the specific surface area thereof, a rapid increase in the crystal size thereof, and a decrease in the oxygen storage capability and oxygen mobility thereof. That is, ceria has a problem of low heat resistance.